The present invention relates generally to holding apparatuses, and more particularly to a holder for holding an optical element used in an exposure apparatus that exposes an object such as a single crystal plate for a semiconductor wafer or a glass plate for a liquid crystal display (“LCD”). The present invention is particularly suitable, for example, for a holder for holding an optical element used in an exposure apparatus that uses ultraviolet light or extreme ultraviolet light (“EUV”) as an exposure light source.
To transfer the circuit pattern, a reduction projection exposure apparatus, which uses a projection optical system, has conventionally been employed to project a circuit pattern formed on a mask (“reticle”) onto a wafer, etc, when manufacturing fine semiconductor devices such as a semiconductor memory or a logic circuit with photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. Therefore, the shorter the wavelength, the better the resolution. Along with recent demands for finer semiconductor devices, shorter wavelengths of ultraviolet light, from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm), has been proposed.
However, lithography using ultraviolet light has limitations when it comes to satisfying the rapidly promoted fine processing of semiconductor devices. Therefore, to efficiently transfer a very fine circuit pattern of 100 nm or less, a reduction projection optical system using extreme ultraviolet (“EUV”) light with a wavelength of 10 to 15 nm shorter than that of ultraviolet has been developed.
However, as the wavelength of the exposure light becomes shorter, the light absorption in a material increases remarkably. Thus, this makes it difficult to use a refraction element or lens used for visible light and ultraviolet light. In addition, no glass material exists for the EUV light's wavelength range. Furthermore, a reflection-type or catoptric optical system uses only a reflective element or mirror (for example, a multilayer mirror).
Therefore, the mirror does not reflect all the exposure light, but absorbs 30% or more of the exposure light. The absorbed exposure light causes residual heat, deforms the surface shape of the mirror, and deteriorates its optical performance, in particular, imaging performance. Therefore, the mirror is made of a low thermal expansion glass, for example, one having a coefficient of linear expansion of 10 ppb, so as to reduce a mirror's shape change as the temperature changes.
The EUV exposure apparatus, used for exposure of a circuit pattern of 0.1 μm, has strict critical dimension accuracy requirements. Therefore, the permitted mirror's surface shape deformation is only about 0.1 nm or less. As a result, even a mirror with a coefficient of linear expansion of 10 ppb would cause a gradual temperature rise and change the mirror's surface shape. For example, when the mirror has a thickness of 50 mm, a temperature rise of 0.2° C. changes the mirror's surface shape by 0.1 nm. Therefore, the mirror in the EUV exposure apparatus should be maintained at a very precise, constant temperature.
However, to prevent reflectance decrease caused by contaminations adhering to the surface of the mirror due to reactions of residual gas (high polymeric organic gas, etc.) contained in the exposure light path with the EUV light, the EUV exposure apparatus maintains its exposure light path in a high vacuum atmosphere of approximately 1×10−6 Pa. Accordingly, the mirrors are cooled by thermal conduction or heat radiation, instead of convection like a gas blow.
FIG. 9 shows a schematic structure of a mirror cooling apparatus using a liquid or a gas as a coolant. FIG. 9A is a bottom view of the mirror cooling apparatus. FIG. 9B is a side view of the mirror cooling apparatus. Numerical reference 1001 is a mirror formed from a low thermal expansion glass with a low coefficient of thermal conductivity. Numerical references 1002 is a pipe for flowing the coolant, 1003 is a joint for connecting the pipe 1002 to a flow path 1004 (explained after), and 1004 is the flow path to inflow the coolant into the pipe 1002. The pipe 1002 contacts the backside of the mirror 1001. The mirror cooling apparatus cools the mirror 1001 by thermal conduction heat transfer with the coolant that flows in the pipe 1002. Therefore, the mirror's 1001 temperature rise can be controlled. FIG. 10 is a schematic structure of a mirror cooling apparatus using heat transfer by radiation. The pipe 1002 is arranged to a radiation plate 1005 that has an area equal to a bottom surface of the mirror 1001. By using radiation heat transfer between the mirror 1001 and the radiation plate 1005, the mirror cooling apparatus can cool the mirror 1001, control the mirror's 1001 temperature rise, prevent conduction of vibration from the pipe to the mirror, and control the deterioration of optical performance.
A projection optical system uses a plurality of optical elements such as mirrors or lenses in the exposure apparatus. Japanese Patent Publication Application No. 2001-343576 discloses an optical element holder which fixes the optical element to a holding element by supporting the optical element elastically at three points. It can hold the optical element without overstressing, and therefore, decrease the optical element's deformation generated from temperature change or during assembly.
The optical element's temperature rises when it absorbs part of the exposure energy. However, because a conventional holding apparatus cannot withdraw all of the heat from the optical element to the cooling apparatus, but conducts a part of the heat to the holding element via a connection part, the desired optical performance cannot be acquired since the temperature distribution of the optical element will be uneven.
In a case, like that disclosed in Japanese Patent Publication Application No. 10-206714, where the holding apparatus holds the optical element by the holding element, using a linear motor, etc., without contact, the heat is transferred by radiation despite the vacuum state. However, as explained before, the temperature distribution of the optical element will be uneven, particularly due to the heat generation of the linear motor's coil.